High strength hot rolled steel sheet

ABSTRACT

There is provided a high strength hot rolled steel sheet including a predetermined chemical composition. A structure includes, by area ratio, 80% or more of polygonal ferrite, a total of 5% or less of martensite and austenite, and a total of 5% or less of pearlite and cementite, and a remainder is at least one selected from bainitic ferrite and bainite. When a standard deviation of micro-hardness of 50 arbitrary pieces of the polygonal ferrite present within a range of ±100 μm from a central plane in a sheet thickness direction is σHV, the σHV is 30 or smaller. A grain of the polygonal ferrite contains 5×10 7  pieces/mm 2  or more of Ti-containing carbide, and in 50% or more of the Ti-containing carbide, the aspect ratio which is a ratio of a length of a long side to a length of a short side is less than 3. The tensile strength is 540 MPa or higher.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a hot rolled steel sheet, andparticularly relates to a high strength hot rolled steel sheet which hasexcellent hole expansibility and is suitable for chassis components andthe like of automobiles formed into various forms through pressing orthe like.

RELATED ART

Hot rolled steel sheets are manufactured at relatively low cost and arewidely used for various types of industrial equipment includingautomobiles. Recently, from a viewpoint on the restriction on carbondioxide emission entailing the measures against global warming, fuelefficiency of automobiles has been required to be improved. Moreover,for the purpose of reducing weight and ensuring collision safety ofvehicle bodies, high strength hot rolled steel sheets have been widelyapplied to automobile components.

Needless to mention, steel sheets for automobile components have tosatisfy not only the strength but also various types of workability suchas press formability and weldability required at the time of forming thecomponents. For example, when chassis components are press-formed, thefrequency of use of stretch flange forming and burring forming isextremely high. Therefore, high strength hot rolled steel sheets for thechassis components are required to have excellent hole expansibility. Inaddition, in the chassis components, from a viewpoint on ensuringsafety, many components are required to avoid plastic deformation evenin a case where a large load is applied. Therefore, steel sheets for thechassis components are required to have a high yield ratio.

Generally, in high strength hot rolled steel sheets, in order to achieveboth the high yield ratio and the excellent hole expansibility, it isexamined that the structure is uniformly strengthened by controlling thesteel structure to be a single phase structure containing any one offerrite, bainitic ferrite, bainite, and the like, through solid solutionstrengthening of Mn, Si, and the like, and/or carbide of Ti, Nb, V, andthe like or precipitation strengthening due to Cu.

For example, Patent Document 1 discloses a technology that relates to ahigh strength hot rolled steel sheet having excellent holeexpansibility, in which Ti carbide including Mo is dispersed in asubstantially single phase structure of ferrite in a uniform and finemanner. However, in the technology of Patent Document 1, it is essentialto add Mo which is a very expensive alloying element. Therefore, from aneconomic viewpoint, the configuration is not suitable for massproduction.

Patent Document 2 discloses a technology in which elongation and stretchflangeability of a high strength hot rolled steel sheet are improved byappropriately controlling cooling of Ti-added steel containingpredetermined amounts of Mn and Si during a period from hot rolling tocoiling such that a structure having ferrite and bainite is achieved,and causing TiC to be finely precipitated. However, in Patent Document2, there is no consideration for the yield ratio which is one ofcharacteristics necessary for a hot rolled steel sheet applied tochassis components. In addition, even though bainite has a low yieldratio compared to ferrite after precipitation strengthening, thetechnology of Patent Document 2 allows bainite to be included up to 50%,and it is analogized that a high yield ratio cannot be maintained.Moreover, the definition of ferrite defined in Patent Document 2 isunclear, and it is assumed that the ferrite includes so-called bainiticferrite or pseudo-polygonal ferrite which is not polygonal ferrite. Asthe reason, in Patent Document 2, a temperature range of 720° C. orlower at which polygonal ferrite is not sufficiently formed is alsoallowed as a first cooling stop temperature. Bainitic ferrite andpseudo-polygonal ferrite have a structure indicating a yield ratio lowerthan that of polygonal ferrite.

Patent Document 3 discloses a Ti-added high strength hot rolled steelsheet of which toughness and hole expansibility are improved by reducingthe Mn content and controlling the percentage of C which is precipitatedas cementite. However, in the hot rolled steel sheet of Patent Document3, in a case of postulating application to chassis components, forexample, a high yield ratio of 75% or more is not obtained in highstrength steel of 540 MPa or higher.

In addition, Patent Document 4 discloses a technology that relates to ahigh strength hot rolled steel sheet having excellent holeexpansibility, in which corsening of TiC is suppressed by reducing theMn content and the Si content and adding certain amounts of Ti and B.However, since B has an effect of suppressing the recrystallization ofaustenite from being recrystallized, in a case of being subjected tomultiple addition together with Ti having a similar effect, a rollingload during hot rolling increases remarkably, resulting in an increaseof a load to a hot rolling mill. Therefore, there is concern that thetechnology of Patent Document 4 causes operational trouble. In addition,since the strength of a final product considerably varies when the Bcontent fluctuates only by several ppm, steel essentially containing Bis not suitable for mass production.

Patent Document 5 discloses a high strength hot rolled steel sheet whichhas a high yield ratio and excellent hole expansibility and is obtainedby cooling steel containing large amounts of Si, Mn, and Ti under anappropriate cooling condition and causing the structure to be a singlephase structure of granular bainitic ferrite. However, in the technologyof Patent Document 5, in order to obtain the granular bainitic ferritestructure, large amounts of Si and Mn need to be contained, therebyleading to a problem of an increase in alloying cost.

PRIOR ART DOCUMENT [Patent Document]

[Patent Document 1] Japanese Unexamined Patent Application, FirstPublication No. 2002-322540

[Patent Document 2] Japanese Unexamined Patent Application, FirstPublication No. 2007-009322

[Patent Document 3] Japanese Unexamined Patent Application, FirstPublication No. H10-287949

[Patent Document 4] Japanese Unexamined Patent Application, FirstPublication No. 2012-026032

[Patent Document 5] Japanese Unexamined Patent Application, FirstPublication No. 2004-307919

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The present invention has been made in consideration of the currentcircumstances described above. An object of the present invention is toprovide a high strength hot rolled steel sheet having a high yield ratioand excellent hole expansibility. The high strength in the presentinvention indicates that tensile strength (TS) is 540 MPa or higher.

Means for Solving the Problem

Ti is relatively inexpensive and exhibits remarkable precipitationstrengthening with a minute amount of Ti content. In order to achieveexcellent hole expansibility, the inventors have examined structures ofhot rolled steel sheets on the premise that polygonal ferrite isemployed as a main constituent. Moreover, in order to improve thestrength of the structure having excellent hole expansibility andcontaining polygonal ferrite as a main constituent, the inventors haveexamined utilization of precipitation strengthening of Ti. The inventorshave also intensively examined a technique of improving the holeexpansibility in a Ti-containing high strength hot rolled steel sheet inwhich Ti precipitates are precipitated in polygonal ferrite as a mainconstituent of the structures. As a result, the following knowledge hasbeen acquired.

The inventors have measured micro-hardness of each ferrite grain insteel having polygonal ferrite as a main constituent of the structure.As a result, it has been found that the hardness significantly variesdepending on each of the measured grains. Furthermore, it has been foundthat hole expansibility can be remarkably improved by reducingunevenness in the hardness of ferrite grains.

In addition, the inventors have observed the intragranular state ofpolygonal ferrite of a sample having inferior hole expansibility, usinga transmission electron microscope. As a result, it has been found thata large amount of Ti-based anisometric carbide stretched in a particularorientation of the ferrite is precipitated and this exerts an adverseinfluence on the hole expansibility. In the related art, there have beenfew reports on the shape of Ti carbide affecting hole expansibility, andthe mechanism of the shape of Ti-based carbide affecting holeexpansibility is obscure. However, compared to Ti-based isometriccarbide, Ti-based anisometric carbide is highly consistent with matrixphase ferrite, and it is estimated that considerable consistencydistortion is accumulated around the Ti-based anisometric carbide.Therefore, it is estimated that this consistency distortion incitescracks to be propagated during hole expanding resulting in deteriorationof the hole expansibility.

The present invention has been made based on the knowledge describedabove. The gist is as follows.

(1) According to an aspect of the present invention, there is provided ahot rolled steel sheet including, as a chemical composition, by mass %,C: 0.010% to 0.200%, Si: 0.001% to 2.50%, Mn: 0.001% to 1.50%, P: 0.050%or less, S: 0.010% or less, N: 0.0070% or less, Al: 0.001% to 0.50%, Ti:0.050% to 0.30%, V: 0% to 0.50%, Nb: 0% to 0.090%, Cr: 0% to 0.50%, Ni:0% to 0.50%, Cu: 0% to 0.50%, Mo: 0% to 0.50%, B: 0% to 0.0050%, Ca: 0%to 0.01%, Mg: 0% to 0.01%, Bi: 0% to 0.01%, and a remainder of Fe andimpurities. The structure includes, by area ratio, 80% or more of apolygonal ferrite, a total of 5% or less of a martensite and anaustenite, and a total of 5% or less of a pearlite and a cementite, andthe remainder is at least one selected from a bainitic ferrite and abainite. When a standard deviation of micro-hardness of 50 arbitrarypieces of the polygonal ferrite present within a range of ±100 μm from acentral plane in a sheet thickness direction is σHV, the σHV is 30 orsmaller. A grain of the polygonal ferrite contains 5×10⁷ pieces/mm² ormore of Ti-containing carbide, and in 50% or more of the Ti-containingcarbide, the aspect ratio which is a ratio of a length of a long side toa length of a short side is less than 3. The tensile strength is 540 MPaor higher.

(2) In the hot rolled steel sheet according to (1) the chemicalcomposition may include, by mass %, at least one selected from the groupconsisting of V: 0.010% to 0.50%, Nb: 0.001% to 0.090%, Cr: 0.001% to0.50%, Ni: 0.001% to 0.50%, Cu: 0.001% to 0.50%, Mo: 0.001% to 0.50%,and B: 0.0001% to 0.0050%.

(3) In the hot rolled steel sheet according to (1) or (2), the chemicalcomposition may include, by mass %, at least one selected from the groupconsisting of Ca: 0.0001% to 0.01%, Mg: 0.0001% to 0.01%, and Bi:0.0001% to 0.01%.

(4) The hot rolled steel sheet according to any one of (1) to (3) mayfurther include a hot-dip galvanized layer on a surface.

Effects of the Invention

According to the aspect of the present invention, it is possible toinexpensively manufacture a high strength hot rolled steel sheet havinga high yield ratio and excellent hole expansibility. In addition, thesteel sheet according to the aspect of the present invention also hasexcellent hole expansibility even during stretch flanges formingfrequently employed for automobile components, particularly chassiscomponents and the like. Therefore, the steel sheet particularlycontributes to reducing weight and ensuring collision safety of vehiclebodies in automobile fields.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing an example of a treatment pattern ofhot rolling.

FIG. 2 is a schematic view showing an example of a heat treatmentpattern in a galvannealing line employed in Example 2.

FIG. 3A is a view showing an example of micro-hardness distribution ofpolygonal ferrite measured in Example 1.

FIG. 3B is a view showing another example of micro-hardness distributionof polygonal ferrite measured in Example 1.

EMBODIMENT OF THE INVENTION

Hereinafter, a high strength hot rolled steel sheet according to anembodiment of the present invention (hereinafter, will be sometimesreferred to as a hot rolled steel sheet according to the presentembodiment) will be described in detail.

There is provided a hot rolled steel sheet according to the presentembodiment.

(a) The hot rolled steel sheet according to the present embodimentincludes, as a chemical composition, by mass %, C: 0.010% to 0.200%, Si:0.001% to 2.50%, Mn: 0.001% to 1.50%, P: 0.050% or less, S: 0.010% orless, N: 0.0070% or less, Al: 0.001% to 0.50%, and Ti: 0.050% to 0.30%.Furthermore, as necessary, the hot rolled steel sheet includes at leastone selected from the group consisting of V: 0.50% or less, Nb: 0.090%or less, Cr: 0.50% or less, Ni: 0.50% or less, Cu: 0.50% or less, Mo:0.50% or less, B: 0.0050% or less, Ca: 0.01% or less, Mg: 0.01% or less,and Bi: 0.01% or less; and a remainder of Fe and impurities.

(b) A structure includes, by area ratio, 80% or more of polygonalferrite, a total of 5% or less of martensite and austenite, and a totalof 5% or less of pearlite and cementite, and the remainder is at leastone selected from bainitic ferrite and bainite.

(c) When a standard deviation of micro-hardness of 50 arbitrary piecesof the polygonal ferrite present within a range of ±100 μm from acentral plane in a sheet thickness direction is σHV, the σHV is 30 orsmaller.

(d) A grain of the polygonal ferrite contains 5×10⁷ pieces/mm² or moreof Ti-containing carbide, and in 50% or more of the Ti-containingcarbide, an aspect ratio which is a ratio of the length of a long sideto the length of a short side is less than 3.

(e) Tensile strength is 540 MPa or higher.

<Chemical Composition of Steel Sheet>

First, the reasons for limiting the chemical composition of the hotrolled steel sheet according to the present embodiment will bedescribed. Hereinafter, all the percentage signs “%” regulating thechemical composition indicate “mass %”.

[C: 0.010% to 0.200%]

C is an element essential to high-strengthening of a steel sheetperformed through precipitation strengthening or solid solutionstrengthening. In order to achieve this effect, the C content is set to0.010% or more, is preferably set to 0.020% or more, and is morepreferably set to 0.040% or more. Meanwhile, when there is an excessiveamount of C, forming of polygonal ferrite is suppressed and cementite islikely to be formed. In addition, the hardness difference among thegrains of polygonal ferrite tends to increase. As a result, holeexpansibility deteriorates. In addition, weldability also deterioratesremarkably. Therefore, the C content is set to 0.200% or less, ispreferably set to 0.130% or less, and is more preferably set to 0.110%or less.

[Si: 0.001% to 2.50%]

Si is a solid solution strengthening element and is an element effectivein high-strengthening of a steel sheet. In order to achieve this effect,the Si content is set to 0.001% or more, is preferably set to 0.01% ormore, and is more preferably set to 0.04% or more. Meanwhile, if thereis an excessive amount of Si, island-shaped scale is generated, andsurface quality deteriorates. Therefore, the Si content is set to 2.50%or less, is preferably set to 1.30% or less, and is more preferably setto 0.80% or less.

[Mn: 0.001% to 1.50%]

Mn is an element effective in improving strength of a steel sheet. Inaddition, Mn is an element which fixes S in steel as MnS and suppresseshot embrittlement caused by a solid solution S. In order to achievethese effects, the Mn content is set to 0.001% or more, is preferablyset to 0.10% or more, and is more preferably set to 0.45% or more.Meanwhile, when there is an excessive amount of Mn, ferritictransformation from austenite is delayed, so that it becomes difficultto obtain 80 area % or more of polygonal ferrite, and hole expansibilitydeteriorates. Therefore, the Mn content is set to 1.50% or less, ispreferably set to 1.00% or less, and is more preferably set to 0.80% orless.

[P: 0.050% or Less]

P is an element contained as impurities, which cause weldability andtoughness of a steel sheet to deteriorate. Therefore, it is preferableto have a small amount of P. However, in a case where the P contentexceeds 0.050%, the influence described above becomes prominent.Accordingly, as a range in which deterioration of weldability andtoughness is not prominent, the P content is set to 0.050% or less, ispreferably set to 0.020% or less, and is more preferably set to 0.010%or less.

[S: 0.010% or Less]

S is an element contained as impurities, forming MnS in steel andcausing hole expansibility of a steel sheet to deteriorate. Therefore,it is preferable to have a small amount of S. However, in a case wherethe S content exceeds 0.010%, the influence described above becomesprominent. Accordingly, as a range in which deterioration of holeexpansibility is not prominent, the S content is set to 0.010% or less,is preferably set to 0.0050% or less, and is more preferably set to0.0020% or less.

[N: 0.0070% or Less]

N is an element contained as impurities, forming coarse nitride in steeland causing hole expansibility of a steel sheet to deteriorateremarkably. Therefore, it is preferable to have a small amount of N.However, in a case where the N content exceeds 0.0070%, the influencedescribed above becomes prominent. Accordingly, as a range in whichdeterioration of hole expansibility is not prominent, the N content isset to 0.0070% or less and is preferably set to 0.0050% or less.

[Al: 0.001% to 0.50%]

Al is an element effective in deoxidation of steel. In order to achievethis effect, the Al content is set to 0.001% or more. Meanwhile,although the Al content exceeds 0.50%, not only the effect is saturatedbut also a cost increase is caused. Therefore, the Al content is set to0.50% or less, is preferably set to 0.20% or less, and is morepreferably set to 0.10% or less.

[Ti: 0.050% to 0.30%]

Ti is an element forming carbide in steel and inducing uniformprecipitation strengthening of ferrite. In addition, Ti is also anelement having effects of reducing the amount of a solid solution C bybeing precipitated as TiC and inhibiting precipitation of cementitewhich causes deterioration of hole expansibility. Therefore, in the hotrolled steel sheet according to the present embodiment, Ti is aparticularly important element. When the Ti content is less than 0.050%,the effect is not sufficient. Accordingly, the Ti content is set to0.050% or more, is preferably set to 0.100% or more, and is morepreferably set to 0.130% or more. Meanwhile, if the Ti content exceeds0.30%, toughness deteriorates remarkably and an unnecessary costincrease is caused. Therefore, the Ti content is set to 0.30% or less,is preferably set to 0.25% or less, and is more preferably set to 0.20%or less.

Basically, the hot rolled steel sheet according to the presentembodiment contains the chemical composition described above and theremainder of Fe and impurities. However, in order to improve strengthand hole expansibility, in place of a part of Fe, within the rangedescribed below, the hot rolled steel sheet may further include at leastone selected from the group consisting of V, Nb, Cr, Ni, Cu, Mo, B, Ca,Mg, and Bi. However, since these elements are not necessarily contained,their lower limits are 0%. Here, impurities denote componentsincorporated due to raw materials such as ores and scraps, or otherfactors when steel is industrially manufactured.

[V: 0.010% to 0.50%]

Similar to Ti, V is an element forming carbide in steel. In addition, Vis an element of which the solubility product in austenite is greaterthan that of Ti and which is effective in high-strengthening of a steelsheet. Therefore, although it is expensive compared to Ti, V may becontained as necessary. When the V content is less than 0.010%, theeffect described above cannot be sufficiently obtained. Accordingly, ina case of obtaining the effect described above, the V content is set to0.010% or more, is preferably set to 0.070% or more, and is morepreferably set to 0.140% or more. Meanwhile, when there is an excessiveamount of V, a cost rise is caused. Therefore, even in a case where V iscontained, the V content is set to 0.50% or less.

[Nb: 0.001% to 0.090%]

Similar to Ti, Nb is an element which forms carbide in steel and iseffective in high-strengthening of a steel sheet. Therefore, although itis expensive compared to Ti, Nb may be contained as necessary. When theNb content is less than 0.001%, the effect described above cannot besufficiently obtained. Accordingly, in a case of obtaining the effectdescribed above, the Nb content is set to 0.001% or more. Meanwhile,when there is an excessive amount of Nb, plastic anisotropy of a steelsheet increases, and hole expansibility deteriorates. Therefore, even ina case where Nb is contained, the Nb content is set to 0.090% or less.

[Cr: 0.001% to 0.50%]

[Ni: 0.001% to 0.50%]

[Cu: 0.001% to 0.50%]

[Mo: 0.001% to 0.50%]

[B: 0.0001% to 0.0050%]

All of Cr, Ni, Cu, Mo, and B are elements effective inhigh-strengthening of a steel sheet. Therefore, as necessary, theelements may be contained independently, or two or more thereof may becontained multiply. In order to achieve the effect described above,there is a need to include Cr: 0.001% or more, Ni: 0.001% or more, Cu:0.001% or more, Mo: 0.001% or more, and B: 0.0001% or more. Meanwhile,similar to Mn, these elements delay ferritic transformation after hotrolling. Therefore, when there are excessive amounts of the elements, itbecomes difficult to obtain, by area ratio, 80% or more of polygonalferrite in the structure of a hot rolled steel sheet, and holeexpansibility of a hot rolled steel sheet deteriorates. Therefore, evenin a case where the elements are contained, the amounts thereof are setto Cr: 0.50% or less, Ni: 0.50% or less, Cu: 0.50% or less, Mo: 0.50% orless, and B: 0.0050% or less respectively; and are preferably set to Cr:0.20% or less, Ni: 0.20% or less, Cu: 0.20% or less, Mo: 0.09% or less,and B: 0.0040% or less, respectively.

[Ca: 0.0001% to 0.01%]

[Mg: 0.0001% to 0.01%]

[Bi: 0.0001% to 0.01%]

Ca and Mg are elements contributing to fine dispersion of inclusions insteel. Bi is an element mitigating micro-segregation of substitutionaltype alloying elements such as Mn and Si in steel. All of the elementscontribute to improvement of hole expansibility of a steel sheet.Therefore, as necessary, the elements may be contained independently, ortwo or more thereof may be contained multiply. In order to achieve theeffect described above, each of the elements needs to be contained0.0001% or more. Meanwhile, when there are excessive amounts of theseelements, ductility deteriorates. Therefore, even in a case where theelements are contained, the amounts of the elements are set to 0.01% orless.

<Structure of Hot Rolled Steel Sheet>

Next, the reasons for limiting the structure of the hot rolled steelsheet according to the present embodiment will be described.

[Area Ratio of Polygonal Ferrite: 80% or More]

Polygonal ferrite has a structure effective in improving holeexpansibility. In order to ensure hole expansibility, the area ratio ofpolygonal ferrite is set to 80% or more, is preferably set to 90% ormore, and is more preferably set to 95% or more. The area ratio ofpolygonal ferrite may be 100%. That is, the hot rolled steel sheetaccording to the present embodiment may be constituted of single phasepolygonal ferrite.

[Total Area Ratio of Martensite and Austenite: 5% or Less]

If the area ratio of martensite and austenite exceeds 5% in total, holeexpansibility deteriorates remarkably. Therefore, the total area ratioof the martensite and austenite is set to 5% or less and is preferablyset to 2% or less. In addition, the total area ratio may be 0% (that is,none of martensite and austenite are contained). In addition, theaustenite mentioned herein is so-called residual austenite.

[Total Area Ratio of Pearlite and Cementite: 5% or Less]

If the area ratio of pearlite and cementite exceeds 5% in total, holeexpansibility deteriorates remarkably. Therefore, the total area ratioof the pearlite and cementite is set to 5% or less, is preferably set to3% or less, and is more preferably set to 1% or less. In addition, thetotal area ratio may be 0% (that is, none of the pearlite and cementiteare contained).

[Structure of Remainder]

The structure of the remainder other than those described above includesat least one selected from bainitic ferrite and bainite. However, in acase where the total area ratio of the structure described above is100%, none of bainitic ferrite and bainite are included.

After a structure of a sample cut out from a hot rolled steel sheet isrevealed through etching, the structure described above can beidentified from a photograph of the structure.

Polygonal ferrite formed by a diffusion mechanism has no internalstructures in grains, and its grain boundary is linear or forms an arc.Meanwhile, bainitic ferrite and bainite have an internal structure, havean acicular intergranular shape, and have a structure distinctlydifferent from that of polygonal ferrite. Therefore, polygonal ferrite,bainite, and bainitic ferrite can be determined based on theintergranular shape and the presence or absence of the internalstructure from a photograph of the structure obtained by using anoptical microscope after etching performed with nital. In a case wherethe internal structure is not distinctly revealed and a structure havingan acicular intergranular shape (pseudo-polygonal ferrite) is present,it is counted as bainitic ferrite.

In addition, since cementite and pearlite are etched in black, theirstructures can be distinctly discriminated.

In addition, an image analysis is performed with respect to a photographof the structure obtained by means of an optical microscope employing aLe Pera-etched sample, so that the total area ratio of residualaustenite and martensite can be calculated.

In the present embodiment, a structure of a steel sheet is observed at ¼position of the depth in a sheet thickness, in which a representativestructure of the steel sheet is shown.

[Standard Deviation σHV of Micro-hardness of 50 Arbitrary Pieces ofPolygonal Ferrite Present within Range of ±100 μm from Central Plane inSheet Thickness Direction: 30 or Smaller]

As described above, hole expansibility of a hot rolled steel sheet canbe remarkably improved by reducing unevenness in hardness of ferritegrains. Specifically, when a hardness (micro-hardness) of 50 arbitrarypieces of polygonal ferrite present within a range of ±100 μm from acentral plane (a face which includes a central portion of the sheetthickness of a steel sheet and is perpendicularly orthogonal to thesheet thickness direction) in the sheet thickness direction is measured,and when a standard deviation of the micro-hardness is the σHV,excellent hole expansibility can be obtained by setting the σHV to 30 orsmaller. Therefore, the σHV is set to 30 or smaller. Since the standarddeviation is preferred to be small, the lower limit of the σHV is zero.

A specific method of measuring the σHV will be described below. As asample for measuring hardness, a steel sheet of which a cross section ina rolling direction is subjected to mirror polishing and in whichchemical polishing is performed using colloidal silica in order toremove a worked layer on a surface layer and then the grain boundary isrevealed through nital-etching is used. The micro-hardness is measuredusing a micro-hardness measuring apparatus (brand name: FISCHERSCOPE HM2000 XYp) by pushing a pyramidal Vickers indenter having an apex angleof 136° into a grain such that its indentation does not overlap thegrain boundary of ferrite with respect to randomly selected 50 pieces ofpolygonal ferrite (grains) which are present within a range of ±100 μmfrom the central plane in the sheet thickness direction. The indentationload is set to 20 N. The standard deviation σHV of the micro-hardness isobtained from the 50 pieces of obtained data.

[Ti-Containing Carbide Present in Grain of Polygonal Ferrite: 5×10⁷Pieces/mm² or More]

[Aspect Ratio of Long Side/Short Side in 50% or More of Ti-containingCarbide Present in Grain of Polygonal Ferrite, Less than 3]

In the hot rolled steel sheet according to the present embodiment, 5×10⁷pieces/mm² or more of Ti-containing carbide are included in a grain ofpolygonal ferrite. When there are 5×10⁷ pieces/mm² or less,precipitation strengthening is insufficient, thereby resulting instrength deficiency. Meanwhile, there is no need to regulate the upperlimit. Generally, when the number is within the component rangedescribed above, the number does not exceed 1×10¹¹ pieces/mm².

In addition, among the pieces of Ti-containing carbide present in agrain of polygonal ferrite, when 50% or more of the carbide, by thenumber percentage, has the ratio of the length of the short side to thelength of the long side (aspect ratio expressed as long side/short side)less than 3, excellent hole expansibility can be obtained. It ispreferable to include ⅔ or more of Ti-containing carbide having theaspect ratio of long side/short side less than 3 among the Ti-containingcarbide present in a grain of polygonal ferrite. The percentage of theTi-containing carbide having the aspect ratio less than 3 may be 100%.

The percentage of the Ti-containing carbide having the aspect ratio lessthan 3 is obtained by setting orientation of an electron beam to beparallel to <001> of matrix phase ferrite and obtaining carbide havingthe aspect ratio of long side/short side less than 3 with respect to thetotal number of pieces of observed Ti-containing carbide when 100 ormore pieces of Ti-containing carbide are observed using a transmissionelectron microscope (magnification: 200,000-fold).

In the present embodiment, the Ti-containing carbide is carbidecontaining Ti, and the Ti-containing carbide may further contain atleast one of V and Nb. That is, the Ti-containing carbide also includesa state where carbide has a crystal structure (NaCl structure) ofTi-containing carbide and some locations of Ti are substituted with V orNb.

[Hot-Dip Galvanized Layer]

The hot rolled steel sheet according to the present embodiment may havea known hot-dip galvanized layer on its surface. The hot-dip galvanizedlayer may be a galvannealed layer which is alloyed. In a case where asteel sheet has a hot-dip galvanized layer, rust is restrained frombeing generated, and the corrosion resistance of the hot rolled steelsheet is improved.

<Mechanical Characteristics of Steel Sheet>

[Tensile Strength (TS): 540 MPa or Higher]

[Ratio (Yield Ratio) of Tensile Strength (TS) and 0.2% Proof Stress(YS): 75% or More]

[Product (TS·λ) of Tensile Strength (TS) and Hole Expanding Rate (2)Regulated by JFST 1001: 50,000 MPa·% or Higher]

In order to satisfy strict performance required in recent high strengthhot rolled steel sheets for automobiles, as their mechanicalcharacteristics, it is preferable that tensile strength TS is 540 MPa orhigher, the ratio (yield ratio (YR)) of the tensile strength TS and 0.2%proof stress YS is 75% or more, and the product (TSλ) of the tensilestrength TS and a hole expanding rate λ regulated by JFST 1001 is 50,000MPa·% or higher. The hot rolled steel sheet according to the presentembodiment aims to be provided with all the high tensile strength, thehigh yield ratio, and the high balance between the tensile strength andthe hole expansibility (TS·λ) by controlling the chemical compositionand the structure.

The tensile strength is preferably set to 590 MPa or higher. Inaddition, if the tensile strength exceeds 1,180 MPa, fatigue propertiesof a weld portion deteriorate. Accordingly, it is preferable to be 1,180MPa or lower.

Next, a preferable manufacturing method for obtaining the hot rolledsteel sheet according to the present embodiment will be described. Thehot rolled steel sheet according to the present embodiment can be stablymanufactured in accordance with a manufacturing method including thefollowing processes (A) to (D), and it is preferable.

(A) A slab obtained from molten steel having the chemical compositionwithin the range described above is heated to approximately 1,200° C.

(B) The heated slab is subjected to rough rolling such that thecumulative rolling reduction within a range from 1,050° C. to 1,150° C.becomes 50% or larger.

(C) The steel sheet after rough rolling is subjected to finish rollingsuch that the cumulative rolling reduction at 1,050° C. or lower rangesfrom 20% to 80%, the rolling reduction of the last pass ranges from 15%to 35%, and the temperature of the last pass (finishing temperature)becomes 930° C. or higher.

(D) Thereafter, with respect to the hot rolled steel sheet, i) asprimary cooling, cooling is performed under the condition that theaverage cooling rate within the temperature range from a finish rollinglast pass temperature to MT (720° C.≤MT≤830° C.) becomes 30° C./s orfaster. Thereafter, ii) as secondary cooling, cooling is performed for tseconds which is regulated by t(sec)=5·(Mn)² under the condition thatthe average cooling rate within the temperature range from MT to Tx(720° C.≤Tx<MT) (here, (Mn) is the Mn content by unit mass %) becomes10° C./s or slower. Subsequently, iii) as third cooling, cooling isperformed under the condition that the average cooling rate within thetemperature range from Tx to CT (450° C.≤CT≤650° C.) which is asecondary cooling end temperature becomes 30° C./s or faster. Then,after being cooled to CT, the hot rolled steel sheet is coiled.

Hereinafter, the reasons will be described.

<Heating Process>

In the heating process, a slab having a chemical composition asdescribed above is heated to approximately 1,200° C. From viewpoints onaffecting the precipitation density of Ti-containing carbide in a grainof polygonal ferrite, and the solid solution states of carbide formingelements such as Ti, Nb, and V; and restraining coarse carbide frombeing formed, in order to obtain desired performance, it is preferablethat the heating temperature is within the temperature range from 1,150°C. to 1,250° C.

<Rough Rolling Process>

The heated slab becomes a hot rolled steel sheet via the hot rollingprocess including the rough rolling process and the finish rollingprocess. When the hot rolled steel sheet according to the presentembodiment is manufactured, in each process of rough rolling and finishrolling, it is preferable to control the temperature, the rollingreduction, and the like.

In the rough rolling process of hot rolling, it is preferable that thecumulative rolling reduction within the range from 1,050° C. to 1,150°C. is set to 50% or larger. When the cumulative rolling reduction withinthe range from 1,050° C. to 1,150° C. falls short of 50%, the structurebecomes inhomogeneous, and there are cases where the σHV increases andhole expansibility is degraded. The cumulative rolling reduction in thepresent invention is the percentage of the cumulative rolling reductionamount (difference between the inlet sheet thickness before the firstpass in rolling and an outlet sheet thickness after the last pass inrolling) with respect to a reference, while the reference is an inletsheet thickness before a first pass. In addition, the cumulative rollingreduction is calculated in each of rough rolling and finish rolling.That is, the cumulative rolling reduction in rough rolling is thepercentage of the difference between the inlet sheet thickness beforethe first pass in rough rolling and the outlet sheet thickness after thelast pass in rough rolling. The cumulative rolling reduction in finishrolling is the percentage of the difference between the inlet sheetthickness before the first pass in finish rolling and the outlet sheetthickness after the last pass in finish rolling.

<Finish Rolling Process>

In the finish rolling process of hot rolling, it is preferable that thecumulative rolling reduction at 1,050° C. or lower ranges from 20% to80%. if the cumulative rolling reduction at 1,050° C. or lower exceeds80%, the anisotropy of the finally obtained structure of the hot rolledsteel sheet is revealed. In this case, there are cases where the σHVincreases and hole expansibility is degraded. The reason is presumed tobe the hardness difference which is incited by deviation of the crystalorientation of ferrite grains. Meanwhile, if the cumulative rollingreduction at 1,050° C. or lower falls short of 20%, the austenite grainsize is coarsened and accumulation of distortion in austenite becomesinsufficient. Accordingly, ferritic transformation after finish rollingis suppressed, and the finally obtained polygonal ferrite fraction andstandard deviation of micro-hardness of polygonal ferrite deviate fromthe desired range, and the possibility of deterioration of holeexpansibility increases.

[Rolling Reduction of Last Pass: 15% to 35%]

It is preferable that the rolling reduction of the last pass is from 15%to 35%. If the rolling reduction of the last pass exceeds 35%, theanisotropy of the structure is revealed. As a result, there are caseswhere the σHv increases and hole expansibility is degraded. Therefore,the rolling reduction of the last pass is set to 35% or smaller and ismore preferably set to 25% or smaller. Meanwhile, if the rollingreduction of the last pass falls short of 15%, accumulation ofdistortion in austenite becomes insufficient. Accordingly, ferritictransformation after finish rolling is suppressed, and the finallyobtained polygonal ferrite fraction and standard deviation ofmicro-hardness of polygonal ferrite deviate from the desired range, andthe possibility of deterioration of hole expansibility increases.

[Finishing Temperature: 930° C. or Higher]

It is preferable that the finishing temperature (temperature of thesteel sheet after the last pass of finish rolling) is set to 930° C. orhigher. If the finishing temperature falls short of 930° C., theanisotropy of the structure is likely to be revealed in the finallyobtained hot rolled steel sheet. As a result, the σHv increases, and thepossibility of deterioration of hole expansibility increases. Meanwhile,in accordance with an increase of the finishing temperature, theaustenite grain size is coarsened and accumulation of distortion inaustenite becomes insufficient. Accordingly, ferritic transformationafter finish rolling is suppressed, and the finally obtained polygonalferrite fraction and standard deviation of micro-hardness of polygonalferrite grow, so that the possibility of deterioration of holeexpansibility increases. Therefore, it is preferable that the upperlimit of the finishing temperature is set to approximately 1,000° C.

<Cooling Process>

After the finish rolling, the hot rolled steel sheet is subjected tocooling.

Within the temperature range from the finish rolling last passtemperature to 720° C., i), a change in density of the Ti-containingcarbide in a grain of polygonal ferrite due to the growing (coarsening)of Ti-containing carbide precipitated in ferrite, and ii) a change inaspect ratio of long side/short side of the Ti-containing carbidepresent in a grain of polygonal ferrite increase. Therefore, in order toobtain the desired performance, it is effective that the average coolingrate within the temperature range from the finish rolling last passtemperature to 720° C. is set to 30° C./s.

Furthermore, after the cooling, within the temperature range from 830°C. to 720° C., cooling of the hot rolled steel sheet at a low averagecooling rate for a desired time which is determined in accordance withthe Mn content is effective in promoting ferritic transformation andprecipitation of the Ti-containing carbide, and having the finallyobtained polygonal ferrite fraction and standard deviation ofmicro-hardness of polygonal ferrite within the desired range.

Thereafter, cooling is further performed, and then the hot rolled steelsheet is coiled. In this case, if the cooling rate is slower than 30°C./s or the coiling temperature exceeds 650° C., Ti-containing carbidein the hot rolled steel sheet is excessively coarsened during thecooling or after the coiling, and there are cases where it becomesdifficult to ensure the desired strength. Meanwhile, in a case where thecoiling temperature is set to less than 450° C., accuracy of controllingthe coiling temperature is degraded, and it is not preferable.Therefore, in order to be effective, the coiling temperature is set torange from 450° C. to 650° C., and cooling is performed until thetemperature reaches the coiling temperature at a predetermined averagecooling rate or faster.

That is, in the cooling process after finish rolling, with respect tothe hot rolled steel sheet after finish rolling, it is preferable thati) as primary cooling, cooling is performed under the condition that theaverage cooling rate within the temperature range from a finish rollinglast pass temperature to MT (720° C.≤MT≤830° C.) becomes 30° C./s orfaster. Thereafter, ii) as secondary cooling, cooling is performed for tseconds which is regulated by the following Expression 1 under thecooling condition that the average cooling rate within the temperaturerange from MT to Tx (720° C.≤Tx<MT) becomes 10° C./s or slower.Subsequently, iii) as third cooling, cooling is performed under thecooling condition that the average cooling rate becomes 30° C./s orfaster within the temperature range from the secondary cooling endtemperature (Tx) to CT (450° C.≤CT≤650° C.). Then, coiling is performedwithin the temperature range from the 450° C. to 650° C.

(t(sec)=5·(Mn)²)  Expression 1

Here, (Mn) is the Mn content by unit mass %.

In a case where the hot rolled steel sheet according to the presentembodiment is manufactured, as necessary, the following processes may befurther provided.

<Plating Process>

After the coiling process, a hot-dip galvanizing process for hot-dipgalvanizing a hot rolled steel sheet may be provided. It is possible toform a coating layer on a surface of the steel sheet and to improvecorrosion resistance of the steel sheet by performing hot-dipgalvanizing. In addition, after hot-dip galvanizing, a galvannealedlayer may be formed on a surface of the steel sheet by performingalloying. In addition, in this case, in order to suppress degradation ofstrength of the steel sheet, the maximum heating temperature duringannealing before hot-dip galvanizing dipping is preferably set to 800°C. or lower. Other hot-dip galvanizing conditions may comply withroutine procedures.

<Other Processes>

In the hot rolled steel sheet according to the present embodiment, inaccordance with the routine procedure, after the hot rolling process,pickling may be performed. In addition, before pickling or afterpickling, skin pass rolling may be performed for flatness correction orpromotion of scale peeling. The elongation rate in a case of performingskin pass rolling is not particularly regulated. However, it ispreferable to set to range from 0.1% to less than 3.0%.

EXAMPLES

Hereinafter, Examples of the present invention will be described.

Example 1

Pieces of steel respectively having the chemical compositions indicatedin Table 1 were each formed into ingot at a laboratory and were castinto slabs. Then, the slabs were subjected to heating, hot rolling,cooling, and coiling in the pattern as shown in FIG. 1. In this case,the conditions in each process were as indicated in Table 2. In Table 2,SRT, R1, R2, R3, FT, MT, t, and CT indicate the following, respectively.

SRT: slab heating temperature

R1: cumulative rolling reduction within range from 1,050° C. to 1,150°C.

R2: cumulative rolling reduction at 1,050° C. or lower

R3: rolling reduction at last finish pass

FT: finish rolling temperature

MT: primary cooling stop temperature

t: secondary cooling time

CT: coiling temperature

Hot rolled steel sheets obtained as described above were subjected topickling. In regard to the condition indicated as plating in the spacesfor treatment in Table 3, after hot-dip galvanizing was performed, JISNo. 5 tensile test pieces were respectively collected from the hotrolled steel sheets in a direction perpendicular to the rollingdirection. A tensile test was performed using these test pieces, and theyield strength (YS), the tensile strength (TS), the yield ratio (YR),and the total elongation (EL) were measured.

In addition, a hole expanding test was performed based on “JFS T 1001the hole expanding test method” of the Japan Iron and Steel FederationStandard, and the hole expanding rate (λ) was measured.

In addition, samples each including a cross section of the hot rolledsteel sheet in the rolling direction were collected. A surfacecorresponding to the cross section of each sample in the rollingdirection was etched using a nital solution. Thereafter, a photograph ofthe structure obtained in the visual field of 300 μm×300 μm at ¼position of the depth in the sheet thickness was captured using anoptical microscope or an electronic scanning microscope, and thestructure was identified. From the photograph of the obtained structure,the area ratio of each structure was calculated through a point countingmethod. Polygonal ferrite, bainite, and bainitic ferrite were determinedbased on the intergranular shape and the presence or absence of theinternal structure. The structure etched in black was discriminated fromcementite and pearlite. In addition, by means of a Le Pera-etchedsample, an image analysis was performed with respect to the photographof the structure obtained using the optical microscope, and the totalarea ratio of residual austenite and martensite was thereby calculated.

In addition, a pellicle sample was collected from each of the hot rolledsteel sheets. Then, carbide containing at least one of Ti, V, and Nbprecipitated in a grain of ferrite was observed using the transmissionelectron microscope (magnification: 200,000-fold), and the numberdensity and the percentage of the precipitated element having the aspectratio of 3 or less were obtained.

In addition, the standard deviation of micro-hardness of the steel fromwhich 80 area % or more polygonal ferrite could be obtained was measuredthrough the method described above. FIGS. 3A and 3B respectively showthe measurement results of micro-hardness of the sample number 14 andthe sample number 15, as examples.

Tables 3 and 4 show the obtained results. In Tables 3 and 4, Vα, VPθ,VMA, B, BF, and σHV indicate the following, respectively. The blankspaces for the structures denote that no observation was performed.

Vα: area ratio of ferrite

VPθ: total area ratio of pearlite and cementite

VMA: total area ratio of martensite and austenite

B, BF: bainite and bainitic ferrite

σHV: standard deviation of micro-hardness of ferrite

In the sample numbers 1 to 3, 5, 6, 11, 17 to 19, 22, and 25 to 34,since all the chemical compositions and the structures were within therange regulated by the present invention, desired mechanicalcharacteristics were obtained. Meanwhile, in the sample numbers 4, 10,12 to 16, 20 to 21, 24, and 36, the σHV exceeded the upper limitregulated by the present invention. As a result, desired mechanicalcharacteristics could not be obtained. In the sample numbers 7, 8, 18,and 36, the area ratio of polygonal ferrite fell short of the lowerlimit regulated by the present invention. As a result, desiredmechanical characteristics could not be obtained. In the sample number9, the total area ratio of martensite and austenite overtook the upperlimit regulated by the present invention. As a result, desiredmechanical characteristics could not be obtained. In the sample numbers36 and 38, the total area ratio of pearlite and cementite overtook theupper limit regulated by the present invention. As a result, desiredmechanical characteristics could not be obtained.

In addition, in the sample numbers 7, 8, 12, 23, 24, 35, and 38, thenumber density of carbide was low. In addition, in the sample numbers 7,8, 12, 23, 24, and 36, the percentage of the Ti-containing carbidehaving the aspect ratio of 3 or less increased, so that desiredmechanical characteristics could not be obtained.

In the sample number 37, toughness was low and breaking occurred at thetime of test piece processing. Accordingly, no test could be performed.

TABLE 1 Kind Chemical composition (mass %) (remainder of Fe andimpurities) of steel C Si Mn P S Al Ti N Cu Cr Ni A 0.036 0.10 0.510.012 0.0020 0.049 0.120 0.0036 B 0.057 0.05 0.50 0.010 0.0020 0.0450.200 0.0035 C 0.057 0.05 1.14 0.010 0.0020 0.045 0.200 0.0035 D 0.0550.60 0.76 0.010 0.0020 0.048 0.200 0.0033 E 0.050 1.15 1.28 0.009 0.00180.050 0.180 0.0030 F 0.079 0.05 0.51 0.009 0.0015 0.050 0.190 0.0033 G0.094 0.05 0.52 0.011 0.0022 0.044 0.190 0.0029 H 0.057 0.06 1.75 0.0100.0019 0.052 0.200 0.0030 I 0.060 0.05 0.50 0.010 0.0020 0.050 0.2000.0028 J 0.056 0.06 0.51 0.010 0.0020 0.050 0.200 0.0028 0.210 K 0.0510.04 0.50 0.010 0.0020 0.050 0.190 0.0028 0.220 L 0.055 0.05 0.50 0.0100.0020 0.050 0.180 0.0028 0.200 M 0.054 0.05 0.54 0.010 0.0020 0.0500.190 0.0028 N 0.055 0.04 0.51 0.010 0.0020 0.050 0.200 0.0028 O 0.0550.05 0.49 0.010 0.0020 0.050 0.190 0.0028 P 0.053 0.06 0.51 0.010 0.00200.050 0.190 0.0028 Q 0.053 0.06 0.51 0.010 0.0020 0.050 0.180 0.0028 R0.055 0.03 1.21 0.001 0.0025 0.120 0.170 0.0025 S 0.080 0.10 0.90 0.0100.0020 0.050 0.040 0.0028 T 0.002 0.05 0.52 0.001 0.0020 0.040 0.1500.0028 U 0.356 0.06 0.60 0.010 0.0019 0.050 0.170 0.0030 V 0.061 0.080.92 0.010 0.0020 0.038 0.420 0.0028 Kind Chemical composition (mass %)(remainder of Fe and impurities) of steel Mo V Nb B Ca Mg Bi Remarks AExample of invention B Example of invention C Example of invention DExample of invention E Example of invention F 0.130 Example of inventionG 0.250 Example of invention H Comparative Example I 0.040 Example ofinvention J Example of invention K Example of invention L Example ofinvention M 0.150 Example of invention N 0.0007 Example of invention O0.0022 Example of invention P 0.0021 Example of invention Q 0.0018Example of invention R Example of invention S Comparative Example TComparative Example U 0.0003 Comparative Example V Comparative Example

TABLE 2 Primary Secondary Third average average average SRT R1 R2 R3 FTcooling rate MT t cooling rate cooling rate CT Condition (° C.) (%) (%)(%) (° C.) (° C./s) (° C.) (sec) (° C./s) (° C./s) (° C.) a 1,200 82 6420 950 40 780 10 5 20 570 b 1,200 82 64 20 950 40 730 10 5 30 570 c1,200 82 64 20 950 50 620 10 5 30 550 d 1,200 82 64 20 950 50 780 1 5 30570 e 1,200 82 64 20 950 40 730 10 15 20 260 f 1,200 93 12 12 950 40 73010 5 40 590 g 1,200 82 64 20 950 60 770 10 8 20 430 h 1,200 82 64 20 95050 680 1 8 30 570 i 1,200 82 64 20 950 30 820 1 8 30 700 j 1,200 37 6420 950 40 770 10 8 30 570 k 1,200 82 90 20 950 40 780 10 10 30 570 l1,200 82 64 42 950 35 770 10 10 30 570 m 1,200 82 64 20 880 50 780 10 1030 570

TABLE 3 Kind Mechanical characteristics Steel structure Sample of TS YSYR EL λ TS · λ Vα VPθ No. steel Condition (MPa) (MPa) (%) (%) (%) (MPa ·%) (%) (%) 1 A a 630 535 84.9 24.2 124 78120 98 1 2 B a 746 599 80.319.6 102 76241.2 98 3 B b 814 670 82.3 18.2 89 72446 96 4 B c 801 65481.6 18.2 60 48060 75 5 C a 748 600 80.2 20.0 100 74800 98 6 C b 891 76585.9 19.6 73 65043 96 7 C c 935 857 91.7 17.6 52 48620 69 8 C d 890 81591.6 18.0 87 48060 60 9 C e 765 569 74.4 19.0 60 46053 87 10 C f 877 77488.3 17.9 57 49989 82 11 D a 791 650 82.2 19.0 101 79891 99 12 D i 712603 84.7 18.0 66 46992 96 13 D j 802 703 87.7 17.2 61 48922 95 14 D k821 673 82.0 18.9 52 42692 97 15 D l 803 666 82.9 18.6 59 47377 98 16 Dm 780 642 82.3 17.9 52 40560 91 17 E a 842 698 82.9 18.9 93 78306 98 18F a 773 671 86.8 18.6 116 89668 99 19 G a 863 696 80.6 18.2 94 81122 99Steel structure Proportion Number of Ti- density containing of carbidecarbide having (×10⁷ aspect Sample VMA pieces/ ratio of 3 No. (%)Remainder σHV mm²) or less (%) Treatment Remarks 1 B, BF 15 11.0 87Example of invention 2 B, BF 17 11.3 89 Plating Example of invention 3B, BF 24 12.3 90 Example of invention 4 B, BF 34  3.2 47 ComparativeExample 5 B, BF 23  9.2 78 Example of invention 6 B, BF 27 10.2 89Example of invention 7 B, BF —  2.7 29 Comparative Example 8 B, BF — 3.9 33 Comparative Example 9 10 B, BF 34  5.6 78 Comparative Example 10B, BF 34  7.9 82 Comparative Example 11 B, BF 18  8.9 89 Example ofinvention 12 B, BF 34  4.3 35 Comparative Example 13 B, BF 32 10.2 85Comparative Example 14 B, BF 33 10.5 89 Comparative Example 15 B, BF 32 9.5 82 Comparative Example 16 B, BF 31  9.3 76 Comparative Example 17B, BF 20  9.2 85 Example of invention 18 B, BF 18  9.0 90 PlatingExample of invention 19 B, BF 20 12.1 95 Example of invention

TABLE 4 Kind Mechanical characteristics Steel structure Sample of TS YSYR EL λ TS · λ Vα VPθ No. steel Condition (MPa) (MPa) (%) (%) (%) (MPa ·%) (%) (%) 20 G c 939 784 83.5 18.0 50 46950 65 21 G f 920 745 81.0 18.454 49680 78 22 G g 875 739 84.5 18.1 99 86625 98 23 H a 910 835 91.817.6 47 42770 30 24 H h 935 871 93.2 17.9 43 40205 42 25 I a 790 65482.8 19.1 96 75840 99 26 J a 784 639 81.5 19.7 98 76832 99 27 K a 798656 82.2 20.0 91 72618 98 28 L a 763 626 82.0 20.1 104 79352 98 29 M a777 643 82.8 19.5 92 71484 99 30 N a 780 665 85.3 18.4 89 69420 98 31 Oa 742 601 81.0 19.7 118 87556 99 32 P a 741 599 80.8 19.8 116 85956 9933 Q a 743 597 80.3 19.4 117 86931 98 34 R a 762 618 81.1 19.7 98 7467699 35 T a 482 367 76.1 31.0 83 40006 100  36 U a 859 758 88.2 15.0 1815462 47 9 37 V a Breaking occurred at time of test piece processing 38S a 593 449 75.7 24.7 71 42103 84 8 Steel structure Proportion Number ofTi- density containing of carbide carbide having (×10⁷ aspect Sample VMApieces/ ratio of 3 No. (%) Remainder σHV mm²) or less (%) TreatmentRemarks 20 B, BF 34  4.2 23 Comparative Example 21 B, BF 35 10.2 78Comparative Example 22 B, BF 19 10.5 89 Example of invention 23 B, BF — 5.2 53 Comparative Example 24 B, BF 35  4.2 38 Comparative Example 25B, BF 17 11.9 90 Example of invention 26 B, BF 18 10.3 91 PlatingExample of invention 27 B, BF 21  8.7 87 Example of invention 28 B, BF19  9.2 86 Example of invention 29 B, BF 21 12.1 85 Example of invention30 B, BF 23 10.3 83 Example of invention 31 B, BF 17 11.5 92 Example ofinvention 32 B, BF 17  9.5 90 Example of invention 33 B, BF 17 10.0 87Example of invention 34 B, BF 16 11.0 88 Example of invention 35 B, BF 2  0.01 100  Plating Comparative Example 36 B, BF 35  6.3 51Comparative Example 37 Breaking occurred at time of test pieceprocessing Comparative Example 38 B, BF 15  3.8 83 Comparative Example

Example 2

Next, among the pieces of steel having the chemical compositionindicated in Table 1, five kinds of steel (A to C, G, and H) weresubjected to hot rolling and cooling shown in FIG. 1. Thereafter,descaling was performed. Then, without performing cold rolling, heattreatment simulating the galvannealing line having the pattern shown inFIG. 2 was performed using a continuous heat treatment simulator. Inthis case, the conditions in each process were as indicated in Table 5.In Table 5, RA, LTH, DIP, and GA indicate the following, respectively.

RA: maximum heating temperature

LTH: low-temperature retention temperature

DIP: Zn bath temperature

GA: galvannealing temperature

From the hot rolled steel sheets obtained as described above, JIS No. 5tensile test pieces were respectively collected in a directionperpendicular to the rolling direction. A tensile test was performedusing these test pieces, and the yield strength (YS), the tensilestrength (TS), the yield ratio (YR), and the total elongation (EL) weremeasured. In addition, a hole expanding test was performed based on “JFST 1001 the hole expanding test method” of the Japan Iron and SteelFederation Standard, and the hole expanding rate (2) was measured.

In addition, samples each including a cross section of the steel sheetin the rolling direction was collected, and the area ratio of eachstructure was calculated through the same method as that in Example 1.

In addition, a pellicle sample was collected from each of the hot rolledsteel sheets. Then, carbide containing at least one of Ti, V, and Nbprecipitated in a grain of ferrite was observed using the transmissionelectron microscope (magnification: 200,000-fold), and the numberdensity and the percentage of the precipitated element having the aspectratio of 3 or less were obtained. The standard deviation ofmicro-hardness of the steel from which 80 area % or more polygonalferrite could be obtained was measured through the method describedabove.

Table 6 shows the obtained results. In the sample numbers 39 to 42, and44 to 47, since all the chemical compositions and the structures werewithin the range regulated by the present invention, desired mechanicalcharacteristics were obtained. Meanwhile, in the sample number 43, theσHV exceeded the upper limit regulated by the present invention. As aresult, desired mechanical characteristics could not be obtained. In thesample number 48, the area ratio of polygonal ferrite fell short of thelower limit regulated by the present invention. As a result, desiredmechanical characteristics could not be obtained.

TABLE 5 Hot rolling conditions Primary Secondary Third average averageaverage Galvannealing conditions SRT R1 R2 R3 FT MT cooling rate tcooling rate cooling rate CT RA LTH DIP GA Condition (° C.) (%) (%) (%)(° C.) (° C.) (° C./s) (sec) (° C./s) (° C./s) (° C.) (° C.) (° C.) (°C.) (° C.) a’ 1200 82 64 20 950 780 40 10 5 20 570 740 490 460 Absent b’1200 82 64 20 950 780 40 10 5 30 570 740 490 460 530 c’ 1200 82 64 20950 620 50 10 5 30 550 740 490 460 530

TABLE 6 Steel Kind Mechanical characteristics structure Sample of TS YSYR EL λ TS · λ Vα No. steel Condition (MPa) (MPa) (%) (%) (%) (MPa · %)(%) 39 A a’ 615 541 88.0 24.1 124 76260 98 40 A b’ 617 539 87.4 24.2 13080210 98 41 B a’ 738 620 84.0 19.3 114 84132 98 42 B b’ 733 623 85.019.6 110 80630 98 43 B c’ 796 663 83.3 17.8 58 46168 95 44 C a’ 740 63986.4 20.1 113 83620 98 45 C b’ 741 642 86.6 20.2 111 82251 99 46 G a’850 725 85.3 18.3 99 84150 99 47 G b’ 844 729 86.4 18.2 100 84400 99 48H b’ 899 833 92.7 18.4 50 44950 27 Steel structure Proportion Number ofTi- density containing of carbide carbide having (×10⁷ aspect Sample VPβVMA pieces/ ratio of 3 No. (%) (%) Remainder σHV mm²) or less (%)Remarks 39 B, BF 14 10.2 89 Example of invention 40 B, BF 15 11   90Example of invention 41 B, BF 15  9.8 88 Example of invention 42 B, BF16 11.2 92 Example of invention 43 B, BF 34  4.3 46 Comparative Example44 B, BF 22  8.9 88 Example of invention 45 B, BF 23  9.2 86 Example ofinvention 46 B, BF 20  9.2 93 Example of invention 47 B, BF 22  8.9 89Example of invention 48 4 B, BF —  3.2 48 Comparative Example

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to inexpensivelymanufacture a high strength hot rolled steel sheet having a high yieldratio and excellent hole expansibility. In addition, the steel sheetaccording to the present invention also has excellent hole expansibilityeven during stretch flanges forming frequently employed for automobilecomponents, particularly chassis components and the like. Therefore, thesteel sheet industrially contributes to reducing weight and ensuringcollision safety of vehicle bodies particularly in automobile fields.

1-4. (canceled)
 5. A high strength hot rolled steel sheet comprising, asa chemical composition, by mass %, C: 0.010% to 0.200%, Si: 0.001% to2.50%, Mn: 0.001% to 1.50%, P: 0.050% or less, S: 0.010% or less, N:0.0070% or less, Al: 0.001% to 0.50%, Ti: 0.050% to 0.30%, V: 0% to0.50%, Nb: 0% to 0.090%, Cr: 0% to 0.50%, Ni: 0% to 0.50%, Cu: 0% to0.50%, Mo: 0% to 0.50%, B: 0% to 0.0050%, Ca: 0% to 0.01%, Mg: 0% to0.01%, Bi: 0% to 0.01%, and a remainder of Fe and impurities, wherein astructure includes, by area ratio, 80% or more of a polygonal ferrite, atotal of 5% or less of a martensite and an austenite, and a total of 5%or less of a pearlite and a cementite, and the remainder is at least oneselected from a bainitic ferrite and a bainite, wherein when a standarddeviation of micro-hardness of 50 arbitrary pieces of the polygonalferrite present within a range of ±100 μm from a central plane in asheet thickness direction is σHV, the σHV is 30 or smaller, wherein agrain of the polygonal ferrite contains 5×10⁷ pieces/mm² or more ofTi-containing carbide, and in 50% or more of the Ti-containing carbide,an aspect ratio which is a ratio of a length of a long side to a lengthof a short side is less than 3, and wherein a tensile strength is 540MPa or higher.
 6. The high strength hot rolled steel sheet according toclaim 5, wherein the chemical composition includes, by mass %, at leastone selected from the group consisting of V: 0.010% to 0.50%, Nb: 0.001%to 0.090%, Cr: 0.001% to 0.50%, Ni: 0.001% to 0.50%, Cu: 0.001% to0.50%, Mo: 0.001% to 0.50%, and B: 0.0001% to 0.0050%.
 7. The highstrength hot rolled steel sheet according to claim 5, wherein thechemical composition includes, by mass %, at least one selected from thegroup consisting of Ca: 0.0001% to 0.01%, Mg: 0.0001% to 0.01%, and Bi:0.0001% to 0.01%.
 8. The high strength hot rolled steel sheet accordingto claim 6, wherein the chemical composition includes, by mass %, atleast one selected from the group consisting of Ca: 0.0001% to 0.01%,Mg: 0.0001% to 0.01%, and Bi: 0.0001% to 0.01%.
 9. The high strength hotrolled steel sheet according to claim 5, further comprising: a hot-dipgalvanized layer on a surface.
 10. The high strength hot rolled steelsheet according to claim 6, further comprising: a hot-dip galvanizedlayer on a surface.
 11. The high strength hot rolled steel sheetaccording to claim 7, further comprising: a hot-dip galvanized layer ona surface.
 12. The high strength hot rolled steel sheet according toclaim 8, further comprising: a hot-dip galvanized layer on a surface.